From a plasma display panel (PDP), near infrared ray is emitted together with visible light. However, near infrared ray is also utilized in near infrared remote control for household electric appliances such as household-use televisions, air-conditioners, video cassette recorders, etc. and in other communication equipments, whereby a glitch in such instruments may sometimes be caused by near infrared ray from the plasma display panel. In order to avoid such a trouble, it is common to provide a near infrared absorbing film on the front surface (viewing surface) of a plasma display panel to adjust the transmittance of near infrared ray of from 850 nm to 1,100 nm to be e.g. at most 20%.
Such a near infrared absorbing film is usually one having a near infrared absorbing colorant incorporated in a resin film and is bonded on a transparent substrate such as a glass sheet together with another functional film such as an antireflection film or an electromagnetic wave-shielding film, to constitute an optical filter. However, such an optical filter is required to provide, in addition to the near infrared absorbing film, an adhesive layer separately to bond it, and thus it is not necessarily excellent in productivity.
Therefore, in order to omit the near infrared absorbing film, it has been proposed to incorporate a near infrared absorbing colorant in the adhesive layer (e.g. Patent Document 1). As such a near infrared absorbing colorant, various colorants of e.g. phthalocyanine type, diimonium type, polymethine type, metal complex type, squarylium type, cyanine type, indoaniline type, etc. may for example, be mentioned, and among them, a diimonium type colorant may be mentioned as preferred, since the near infrared absorption wavelength region is broad.
However, a diimonium type colorant is usually poor in heat resistance or moisture resistance and is likely to be deteriorated particularly in a case where it is incorporated in an adhesive layer. By such deterioration of the colorant, not only the near infrared absorption ability will decrease, but also an optical property such as visible transmittance will decrease. Therefore, it has been also proposed to prevent such deterioration by using a diimonium type colorant having specific anions (e.g. Patent Document 2).